I mean this in a constructive way: How about just asking him to not grab your wrist so hard so that you can get more reps in just doing the movement? Did you ask him that? If not, maybe you could also try and ask your own person, "Why not?"

At our dojo, while training is often intense, but most especially because training is often intense, both nage and uke can ask their partner to lighten up any time and for any reason. It's not only acceptable, its demanded that no deshi train beyond their safe level and/or whatever level that they are personally comfortable with (all the while accounting for other guidelines). This way, one takes responsibility for their own training - which is not an easy thing to do. I say it is not an easy thing to do because in taking responsibility for our own training we not only have to learn to become independent from someone or some thing else, we also have to learn how to reconcile our habitual ways of responding to things, people, and ideas - such that we can act according to the infinity of choices that are always before us and not fall victim to doing "the only thing we could do."

In my experience, which may be totally different from yours, folks often do not want to ask their partners to lighten up because they feel it to be a sign of weakness. For this reason, not asking folks to lighten up is often connected to making the Other a villain. In blaming some thing on someone else, we hide our own ego attachment to our own will to power. For this reason, it is often quite liberating, at very deep levels of our being, to practice asking folks to lighten up when necessary and/or when wanted (for whatever reason).

On this side of that request, such a thing seems totally impossible and/or unfounded, but once we reconcile enough of our ego to be able to ask such a thing of our training partner we will wonder why we ever had such a hard time making such a request in the first place. In time, we'll also wonder why were ever attaching issues of power and pride to form's training - which is only about a proximity to an ideal and not a measuring of our martial prowess.

Sure, you may ask such a thing of your current partner, and he may see it as a sign that you want to do "fake Aikido" or that you are "weak." However, as he hopelessly struggles to find a way of measuring his power in the utterly false and constructed reality of forms, you will go on practicing the movement more and more, and thus getting closer and closer to the ideal. In the end, you will find both the power of humility and the humility of power. He will find nothing. In fact, most likely, he will just quit when the realization that forms are nothing actually hits him like a ton of bricks.

The key here is to find a way of reconciling our habitual responses to such situations - things that are grounded in our pride, in our fears, and in our ignorance. If we can do that, such "uke" will not only be the person that we can use to refine our technique, such that strong grabs only make our technique more powerful, such "uke" will also come to teach us a whole lot more about ourselves and about our world as we are experiencing (i.e. constructing) it.

Hang in there - we've all passed through this "uke." Actually, we've all passed through lots of them. You can do it too.